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Ordered Molecular Arrays as Templates A New Approach to the Synthesis of Mesoporous Materials.

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Ordered Molecular Arrays as Templates:
A New Approach to the Synthesis of Mesoporous Materials
By Peter Belirens* and Galen D. Stucky
During the past decade, physicists have discovered a new
world in mesoscopic systems, in which classical models collide with quantum effects. It is a world in which electronic
transport becomes a coherent process and any change in
phase leads to a macroscopic change in conductance. Similarly, small changes in atomic structure o r externally applied
electromagnetic fields can cause large changes in absorption
edges, polarization, optical transparency, and general nonlinear response. Using electrooptic interconnects in this dimension offers the possibilities of decreasing processing
times in optical computers, increasing our capabilities to
manipulate and store visual images, and a host of other
potential applications.
The physicist’s and engineer’s approach to the mesoscopic
world[’] has been by way of molecular beam epitaxy (MBE)
and atomic layer epitaxy (ALE) on substrates with an ordered
two-dimensional lattice to fabricate ultra-thin (monolayer)
semiconductor epitaxial layers. Quantum wires in which carriers and electronic wave functions have only one degree of
freedom can be formed in the MBE approach by corrugation
of quantum layers o r by lateral structuring, but are still a
challenge below diameters of about 100
The chemist’s interest in the mesoscopic world has been
equally intense, based on a long-held recognition that ultimately all materials and living organisms have their origins
in the collective assembly of a small number of atoms or
molecules. Whether it be the beginning of life, crystallization, or the natural formation of a complicated biological
nanocomposite like a sea shell, the inherent future properties such as shape, mechanical strength, color, conductivity,
etc. of an extended structure are established very early
as the molecules or atoms assemble and make the critical
transition from few units with nanoscale dimensions to
mesoscopic arrays. It is at this metamorphosis that macroscopic collective properties are defined and have their
birth.
Two chemical approaches to the micro- and mesophase
manipulation of atoms and molecules have been most extensively used. The first, in which considerable progress has
been made, is the assembly of microcluster atomic arrays.
Examples include small chunks of metal[2a1that are solubilized and protected by a coating of ligands, such as
[HNi38Pt,(CO),8]S-~2b1
and [AU,,(PP~,),,,]~’[~‘’as well as
the use of competitive core cluster growth and surface capping (which terminates the cluster growth) of semiconductor
clusters.r31A limitation has been the difficulty in subsequently
ordering these clusters into three-dimensional supramolecular lattices.
A second approach has been to use two-dimensional, structure-directing surfaces and three-dimensional porous media
A.
[*I
Dr. P. Behrens
Fdkultiit fur Chemie der UniversitPt
Postfach 5560, D-W-7750 Konstanz (FRG)
Telefdx: Int. code + (7531)88-3898
Prof. Dr. G. D. Stucky
University of California. Sdnta Barbara, CA (USA)
696
:C VCH Verlu~sges~~llscliqfi
n?bH, W-6940 Wernhebn, 1993
for molecular recognition. The chemical growth from solution of superlattice layered structures, giving a periodic array
of planes of atoms or molecules with confinement in one
dimension (perpendicular to the expitaxial grown layers) has
been beautifully developed by the research groups of Mallouk, Marks, and Katz for applications such as enantioselective synthesis and nonlinear optical devices.[41The separation of the layers in these nanophase materials is generally
less than 20
Microporous phases with three-dimensional framework
structures are now routinely synthesized by using molecular
units or hydrated alkali or alkaline earth cations as templates. Their well-defined pore structures and pore sizes (3 to
13
reflect the template dimensions. Chronologically the
development of this microporous synthesis chemistry has
evolved from zeolites that possess crystalline aluminosilicate
frameworks to pure silicate phases, and more recently to
compounds that exhibit similar framework structures but
have compositions in which Al and/or Si are substituted by
other elements such as Be, B, Ga, Ge, Zn, and P. The aiuminosilicate and silicate microporous solids have found extensive application in the fields of ion exchange (consider the
use of zeolite A as water softener in detergents), desiccation,
sorption (e.g., gas separation and purification), and catalysis. Especially for the latter two applications, the geometric
restrictions imposed by the porous framework are of importance in differentiating between molecules of different size
(size exclusion from intraporous sorption); and in the selectivity of starting materials, transition states, and products of
catalytic processes (“shape-selective catalysis”).[s1
In the last five years, new applications of the precisely defined void structure of zeolite-type microporous solids have
been envisaged. In an approach to “supramolecular architecture”[61and “nanoscale inclusion chemistry”[’] based not
on molecular ensembles but on extended solid state structures, they serve as a versatile host medium for assembling
and maintaining controlled microstructures of quantum dots
and wires,”] organic molecules and polymers,[g1metal carbonyls and organometallic molecules,[sd. lo’ redox systems
and electron-transport chains,‘”] as well as nanosize reaction vessels.[’ This type of host-guest chemistry results in
synthetic nanocomposite materials that can be described as
“expanded” metals and semiconductors, stabilized arrays of
aligned molecules o r spatially organized redox systems for
which applications in optoelectronics,”“. 8b1 nonlinear optics,’’”- 9bl optical data storage and image processingr8”](for
example by spectral hole burning),[”’ and enzyme mimicking[’2bfare conceivable.
There has been a growing interest in the extension of the
microporous molecular sieve synthesis and applications to
mesoscopic dimensions. Typical areas for the application of
mesoscopic zeolite-type structures are in separation (e.g.,
protein separation and selective adsorption of large organic
molecues from waste waters) and catalysis (e.g., processing
of tar sand and of the high distillates of crude oils to valuable
low-boiling products). Another is in the supramolecular as-
A.
A)
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Angew. Chem. Int.
Ed. Engl. 1993, 32, N o . 5
sembly of molecular arrays and polymers for electronic and
optical applications.
Although there have been numerous attempts to carry over
the crystallinity and the well-defined pore structure of zeolites
to the mesoporous regime, the direct synthesis of threedimensional m e s o p ~ r o u s3l~ materials
’
containing cages and
channels with access pores greater than 13 A has until now
been unsuccessful. Other routes to mesoporous solids such as
forming pillars of layered compounds have been investigated.i14-161Although mesoporosity was observed in some of
the systems investigated, the pore size distribution was broader than in microporous zeolites.
The usual technique for preparing zeolites is the hydrothermal treatment of aluminosilicate gels or solutions containing
metal (alkali or alkali earth) or organic ions (e.g., tetraalkylammonium ions) or molecules (e.g., amines). First progress
towards enlarging the pore sizes in zeolite-type compounds
was achieved by changing the composition of the gel from
that of an aluminosilicate. This led to the discovery of the
aluminophosphate VPI-5,1’71 a one-dimensional channel
structure with channel diameters of z 13 A, and of the gallophosphate cloverite!’81 featuring a cavity with a diameter
of about 30 A. Although the windows are about 13 A at their
widest point, their effective kinetic diameter is much smaller
because of the clover-leaf shape of the opening.
The ions and molecules present in the hydrothermally treated gel act as templates during its crystallization; that is, they
direct the formation of a specific zeolite framework and are
usually occluded in the voids of the crystallizing solids. Even
today this template effect is not fully understood. One proposal is that the template molecules change the chemistry of
the crystallizing gel. Organic templates, on the other hand,
may also act merely as “void fillers”, preventing the crystallization of thermodynamically more stable dense phases.
One obvious way to zeolites with larger pores is the use of
large molecular organic templates, which should give large
voids. This approach was, of course, followed by many laboratories but over a long period has not led to a definite
claim of a mesoporous structure.
Recently, researches of Mobil Oil Company[1g.201used a
new concept in the synthesis of porous material which at
long last has led to the first synthesis of mesoporous structures. The templating agent is no longer a single, solvated,
organic molecule or metal ion, but rather a self-assembled
molecular array. This template leads to mesoporous zeolites
with adjustable pore sizes between 16 and > I00 A, covering
well the mesoporous range of greatest interest. These compounds, named MCM-41 (MCM stands for Mobil’s Composition of Matter), possess a hexagonal arrangement of
parallel channels which can be directly visualized by a transmission electron
The arrangement of pores is
very regular, and the pore size distribution measured by absorption is nearly as sharp as that of conventional zeol i t e ~ . “A
~ structural
, ~ ~ ~ ~ model of an MCM-41 material is
given in Figure 1 .
Kresge et al.[20a1and Beck et a1.[20b1
propose that a liquid
crystal templating mechanism is operative in the synthesis of
their mesoporous materials. Typically, the templates they use
are quaternary ammonium ions CnHZn+
,(CH,),N+, which
are also useful as surfactants in detergents. These ions are
known to form micelles in aqueous solutions (Fig. 2). In a
A n w v . Chem. lnt. Ed. E n d . 1993, 32, No. 5
Fig. 1. Schematic representation of the structure of a MCM-41 phase with an
interpore distance of ~ 3 A,5 amorphous wall structure, and hexagonal pores.
picture similar to the recently proposed “can-and-cement’’
model of the crystallization of microporous
these
template molecules are assumed to be ordered in the aqueous
gel; during crystallization, the silicate or aluminosilicate solution present between the template assemblies becomes the
“walls” of the porous solid. Different types of order have
Fig. 2. Schematic representation of the exterior surface of a cylindrical micelle.
The ends of the micelle cylinder are probably best visualized as hemispherical.
been found in surfactant-water liquid crystals, including
lamellar, hexagonal, and cubic phases.r221Accordingly,
MCM-41 materials, which have a hexagonal arrangement of
mesoporous channels are templated by the hexagonal phase.
Strong support for the liquid crystal template mechanism
comes from the discovery of cubic and layered crystallization
products[”. 231 that possibly are templated by the cubic and
lamellar liquid crystal phases. According to the templating
mechanism, the alkyl chain length n influences the pore size
of the MCM-41 material. For example, for n = 12, 14 or 16
the pore size is 30 A, 34 A, and 38 A, respectively.iLg*201
0 VCH Verlugsgesell.~chaftmbH, W-6940 Weinheim, 1993
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Another way to alter the pore size is the addition of
“auxiliary” template molecules (“expanders”) such as
m e ~ i t y l e n e . 201
~ ’ ~These
~
substances can dissolve into the hydrophobic region of the surfactant micelles, whereupon the
micelle diameter and the pore size increases. This templating
chemistry gives the opportunity to produce “tailor-made”
mesoporous solids.
Investigations of the crystallization process of microporous zeolite-type compounds have shown that the “best”
(i.e., the most strongly structure-directing) templates are
rigid organic molecules stiffened by the presence of one or
more ring systems. Regarding the potential flexibility of
micellar arrangements, the order in the walls of the new
mesoporous materials possibly is less strict than in conventional zeolites. Indeed, the atomic structure of the walls of
the mesopores is of particular interest, both to aid understanding the synthesis mechanism and for use after synthesis as a
structure-directing scaffolding for the assembly of molecular
and atomic nanophases within the pores.123]Tilted-angle
electron and X-ray diffraction measurements of Mobil and
at the University of California, Santa Barbara (UCSB) show
no evidence of long-range ordering along the channel walls;
only (hkO) reflections are observed for the hexagonal MCM41 materials, and no reflections at diffraction angles larger
than about 6” 28 (Cu,, radiation) for pore sizes greater 30 A.
The best fit to the experimental X-ray diffraction data of
MCM-41 is obtained with a continuous scattering model
which assumes that the wall structure does not have longrange order. For the hexagonal mesoporous phase the results indicate that the channels are hexagonal (rather than
cylindrical) and that the wall thickness is 8 1.O A. The wall
thickness appears to be relatively constant with increasing
pore size to z 100 A. The structural model shown in Figure 1
is based on these results.
Solid state N M R results confirm the disorder in the framework. The hydroxyl group concentrations and 29Si N M R
chemical shifts depend strongly on the procedure used to
remove the template. After calcination at 550°C only one
type of hydroxyl group is found at a concentration of approximately 1 for every 10 silicon atoms. These hydroxyl groups
can be readily functionalized with (CH,),SiCI (Mobil and
UCSB) and by various other organometallics, including
(CH,),GeCI, (CH,),Ga, (CH,),Zn, and (CH,),Cd (UCSB).
The temperature stability of the pores is also remarkable.
Reflections (hkO) with higher indices than (loo), which the
X-ray analysis confirms are a measure of pore geometry,
vanish upon heating above 800-900 “C; however, the (100)
scattering which defines the occurrence of pore periodicity
can even be observed from samples heated to 1200 “C. Smallangle and long-wavelength neutron diffraction studies at the
National Institute of Science and Technology have confirmed the long-range ordering of the pores observed by the
X-ray and electron diffraction studies, and along with solution N M R should be particularly useful in elucidating the
mechanism of solution templating during synthesis.
Bein and Wu[241from Purdue University have meanwhile
reported the successful intrapore polymerization of aniline
and thiophene derivatives in MCM-41 materials with an interpore spacing of 35 A. Such encapsulated polymers are an
important step towards the synthesis of oriented molecular
wires (compare ref. [9d]).
698
a VCH Verlagsgesellsrhaf~mbH, W-6940 Weinheim, 1993
There is little doubt that the new mesopore derivatives will
make excellent supports with high surface area. Perhaps
more importantly, they offer new horizons both in the synthesis of nanophase composites with the MCM-41 and cubic
phase mesopores reported by Mobil and suggest exciting
possibilities for the synthesis of other mesoporous materials
of different compositions and geometries on account of the
large number of variations of ordered molecular arrays that
are available. In particular they provide a fascinating avenue
to the chemistry of biomineralization and biogenesis, where
nature has very effectively used ordered molecular arrays in
templating inorganic phases. This, of course, has important
consequences for the use of biomimetic synthetic procedures.
In every sense a new solid state chemistry and materials
science has been given to the scientific community by the
Mobil researchers.
German version: Angew. Cheni. 1993, 105,729
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According to the IUPAC definition mesopores possess diameters between
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Inorganic Solids (Eds.: M. Hudson. C. A. C. Sequeira), Kluver, Dordrecht.
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0570-0833~93jOSOS-0698
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Angew. Chem. Int. Ed. Engl. 1993, 32. No. 5
[19] J. S. Beck (Mobil Oil Company), USA 5057296,1991; C. T. Kresge, M . E.
Leonowicz. W. J. Roth, J. C.Vartuli (Mobil Oil Company), US-A 5098 684,
1992: US-A 5 102643,1992; J. S. Beck. C. T. W. Chu, I. D . Johnson. C. T.
Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli (Mobil Oil Company).
US-A 5 108725, 1992.
[20] a ) C. T. Kresge, M. E. Leonowicz. W. J. Roth, J. C. Vartuli, J. S. Beck,
Nuturr 1992, 359. 710-712; b) J. S. Beck. J. C. Vartuli, W. J. Roth, M. E.
Leonowicz, C. T. Kresge, K. D. Schmitt, C. T. W. Chu. D. H. Olson, E. W.
Sheppard, S. B. McCullen, J. B. Higgins, J. L. Schlenker. J. Am. Chem. SOC.
1992. ff4.10834-10843.
Brunner, Zrolitrs 1992, 12, 428.
1211 G 0.
(221 G. J. T. Tiddy, Phm. Rep, 1980, 37, 1.
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0 VCH
[23] A. Monnier,E. Ramli. D. Margolese. Q. Huo, S. Tomikaya, C. Glinka. J. J.
Rush, J. Nicol, P Petroff, C. D. Stucky, presented at the International
Zeolite Symposium, Montreal, Canada, July 1992. G. D. S. acknowledges
support of the Office of Naval Research. the National Science and Technology Center for Quantized Electronics Structures (QUEST), and the
Materials Research Laboratory at the University of Santa Barbara. The
authors also wish to thank the research personnel in the groups of Dave
Olson and Charlie Kresge at Mobil Research Laboratories i n Princeton
and Paulsboro, New Jersey for their assistance and comments
[24] T. Bein, C. G. Wu, presented at the Meeting of the Materials Research
Society, Boston, November 1992; Muter. RES. Soc. Symp. Proc.. in
press.
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